The invention relates to a method for separating a multi-phase mixture into at least one liquid phase and one dry phase with a specified dry matter concentration CTS, using a decanting centrifuge, which includes:
An annular immersion disk that is connected to a shaft at its inside circumference and that exhibits an outside diameter that is smaller than the inside diameter of a centrifuge drum; and
At least one liquor weir with a weir gap arranged at the end face of the centrifuge drum that can be used to draw the liquid phase from the centrifuge drum, and with a tank depth setting device that can be used to set the tank depth xT of the liquid phase that rotates in the centrifuge drum, using the following steps:
a) Start-up of the centrifuge drum to a start-up drum speed nZ,1, and setting the tank depth xT to a start-up tank depth xT,1;
b) Introducing the multi-phase mixture into the rotating centrifuge drum;
c) Drawing off the dry phase through the at least one dry matter discharge opening, and drawing off the liquid phase through the weir gap; and
d) Controlling the tank depth xT corresponding to the dry matter concentration cTS in the drawn off dry phase until a specified nominal dry matter concentration cTS,1 is reached, using the tank depth setting device.
The tank depth is defined as the difference between the outside and inside diameter of the liquid ring that rotates inside the centrifuge drum.
A decanting centrifuge with at least a partial hydraulic transport system, such as is required for carrying out the method, is known from the German Patent No. DE 43 20 265 C2. With this method, a liquid ring between immersion disk and liquor weir is set in the rotating centrifuge drum to a certain fill level, the so-called tank depth, thus generating a hydrostatic pressure through the liquid phase that assists in discharging the dry phase. In addition to, or in place of, the discharge, the hydraulic transport system can also be carried out using the differential speed of rotatable screws.
Essentially, the weir is designed in two parts. A weir disc closes the cylinder-shaped jacket of the centrifuge drum and rotates with said drum. It is provided with at least one passage for discharging liquid from the centrifuge drum. Assigned to the weir plate is a parallel baffle plate that can be moved axially, and that is arranged at the fixed support of the rotatable centrifuge drum. A gap that stretches in the radial direction is created between the rotating weir plate and the fixed baffle plate. The liquid phase is thrown from the centrifuge drum through said gap. The width of the weir gap can be adjusted by the axial movement of the baffle. Reducing the width in the weir gap increases the pressure in the liquid phase such that the increased pressure increases the amount of the dry phase that is pressed out of the centrifuge drum. To some degree, the liquid phase also enters the dry phase, and thus reduces its concentration on dry matter. Conversely, widening the weir gap results in reduced pressure, a reduced hydraulic conveyance, and finally an increase in the dry matter concentration in the dry phase.
Such a liquid weir has proven itself in decanting centrifuges, because it can be adjusted at a rotating centrifuge drum, thus allowing the control of the dry matter concentration via the width of the weir gap. Using the control of the weir gap width allows for a response to changes in concentration and volume of the supplied multi-phase mixture while the process is running.
However, it has been found that the control of the dry matter concentration using the adjustable liquor weir requires an unchanging high energy input for the decanting centrifuge, which is rotating at high speeds. In particular, the high energy consumption is due to the fact that the supplied amount of multi-phase mixture consistently needs to be accelerated from an idle state until it has reached the high angular speed that is impressed using the centrifuge drum.
In the course of the process, the weir position may shift towards the edge, where the baffle plate of the weir can no longer be adjusted. Controlling the dry matter concentration is then no longer possible if the concentration and/or the amount of the supplied multi-phase mixture change significantly. The process must be interrupted and re-started using a drum speed that is determined empirically.
Known from the German Patent No. DE 195 00 600 is a pneumatic liquor weir, where the flow resistance of the liquid phase is increased by blowing pressure gas in the weir gap, thus increasing the tank depth. This design of the liquor weir also allows for controlling the dry matter concentration through adjusting the weir during operation.
Various methods that influence the properties of the separated phases using machine parameters such as the drum speed or the differential speed are recommended in the European Patent Publication No. EP 1 044 723 A1. However, with these methods, the composition of the liquid and dry phases is always at the center of the considerations. The disclosed control of the dry matter concentration via variations of the drum speed requires, however, increased energy input. In addition to the already high energy consumption at a high base speed, frequent braking and accelerating of the drum is very energy-intensive due to the high mass moment of inertia of a loaded decanting centrifuge and the high angular speeds.
A method that would be suitable for operating a decanting centrifuge using an adjustable liquor weir is not disclosed.